1. Field of the Invention
The present invention is directed to a method and apparatus for scanning a sample using a scanning probe microscope, and more particularly, to a method and apparatus of detecting a transition region in a sample and re-scanning the transition region.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. As throughput requirements increase, increased data acquisition speeds are desired, thus making the ability to obtain reliable data a challenge.
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc. AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is coupled to an oscillating actuator or drive 16 that is used to drive probe 14 to oscillate, in this case, at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to cause actuator 16 to drive the probe 14 to oscillate, preferably at a free oscillation amplitude Ao. Probe 14 is typically actuated to move toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by controller 20. The actuator 16 may be coupled to the scanner 24 and probe 14 or may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though the actuator 24 is shown coupled to the probe 14, the actuator 24 may be employed to move sample 22 in three orthogonal directions as an XYZ actuator, i.e., both Z motion, and X-Y scanning motion such as in raster scanning. Still other permutations are possible.
One or more probes may be loaded into the AFM and the AFM may be equipped to select one of several loaded probes. Typically, the selected probe 14 is oscillated and brought into interaction with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 17 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. Commonly, controller 20 generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, for instance, a setpoint phase or frequency may be used.
Commonly, actuator 24 is a piezoelectric tube (often referred to herein as a “piezo tube”) or flexure that is used to generate relative motion between the measuring probe and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube. Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY actuator that drives the probe or sample in a horizontal, or XY plane and a Z actuator that moves the probe or sample in a vertical or Z direction.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In an oscillation “flexural mode” of operation the cantilever oscillates generally about a fixed end. One flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. Typically, amplitude, phase or frequency of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. These feedback signals are then collected, stored, and used as data to characterize the sample.
As metrology applications demand greater throughput, and as the desirability of using SPM in a wide variety of applications requiring sub-micron measurements continues to grow, improvements to data acquisition using SPM have become necessary. Wafer analysis in the semiconductor industry is one key application. In general, chip makers want to measure structures (e.g., lines, vias, trenches, etc.) having critical dimensions (CDs), such as width of interconnect lines, contacts, trenches, etc., that are 90 nm and below. In this regard, “bottom CD” metrology is particularly interesting to semiconductor customers. Semiconductor device manufacturers often fabricate logic elements such as processors and in doing so want to measure the width of the gate structure, a fundamental element of a transistor and the basis for silicon based logic elements. The ITRS (International Technology Roadmap for Semiconductors) specifies that the bottom CD of semi features such as the gate is a very important parameter that must be controlled to within a few nanometers of uncertainty (with the range decreasing every year), or the resultant transistors will not operate as designed. Therefore, the ability to measure bottom CD accurately and precisely can define whether AFM has sufficient value to many potential customers. As a result, improvements in CD-AFM have become necessary.
When analyzing structures at such small scale, the corresponding measurements require uniformity control and must be able to accommodate high volume production environments. In this regard, one advancement has been in the area of automated AFMs which greatly improve the number of samples that may be imaged in a certain time frame by minimizing expert user tasks during operation. Instruments for performing automated wafer measurements are varied, but AFM offers a unique solution by providing, for example, the ability to perform high-resolution multi-dimension (e.g., 3-D) imaging. Some instruments, like the Dimension X automated AFM offered by Veeco Instruments, have proven 200 mm and 300 mm automation platforms.
According to one type of oscillating mode operation particularly applicable to imaging semiconductor samples, known as CD mode, a critical boot-shaped tip is employed to measure critical dimensions of the individual batch processed semiconductor structures. In CD mode, operational parameters of the AFM operating in an oscillating mode are modified to accommodate sharp transitions in topography associated with different device features including, for example, lines, trenches, vias, etc. Notably, CD mode typically provides a two-dimensional servo (the standard Z servo used in an oscillating mode such as TappingMode™ AFM, as well as an X servo to accommodate sharp transitions), essentially maintaining the servo direction substantially orthogonal to the sample surface. FIG. 2 illustrates an exemplary line 30 having flat-to-sidewall transition regions A and B. Transition A shows a rising edge, i.e., an upwardly extending sidewall transition when scanning in the direction marked “X”, while Region B corresponds to a falling edge. Often, features are scanned in both directions as part of trace/re-trace scanning. The rising and falling nature of the transition of the sample reverses for these scans. Note that transitions referenced herein indicate structural transitions (such as a flat-to-sidewall transition of a semiconductor line) as opposed to other types of transitions that may be measured with an AFM, such as material type.
A schematic version of a tip 40 of a probe device is illustrated in FIGS. 3A-3E. Tip 40 includes a boot-shaped distal end 42 having feet portions 44, 46 extending generally orthogonally to a main axis “A” of the tip. Notably, tips such as CD tip 40 are typically batch fabricated from a semiconductor wafer, but any tip suitable for imaging such features could be used. Also, an AFM operating in CD mode may be referred to herein as CD-AFM.
Again, what is shown in FIGS. 3A-3E is only the tip of the probe device scanning a flat-to-sidewall transition of a sample 48, with the base and cantilever of the probe device not shown. Notably, the shape of the CD tip, i.e., substantially “boot shaped”, allows the AFM to sense transitions in the sample surface. However, due to the dynamics of the system, including limitations associated with the scanner, information regarding these transition regions is often difficult to obtain with standard CD mode AFM. Namely, when operating the AFM at high speeds, even if the bandwidth of the servo scanner is sufficient to respond to a command indicative of tip contact with a transition wall, the inertia of the scanner after the system instructs the scanner to stop scanning (in what as shown as the “X” direction) causes the tip 40 to be driven further toward sidewall 54 of sample 48. FIGS. 3A-3E highlight how these limitations results in either biased or lost information regarding sample 48 in the transition region 50.
Referring initially to FIG. 3A, a tip 40 having a distal end 42 is introduced to a flat portion 52 of sample 48 and is scanned in a direction marked “X” along a scan line. In this case, sample 48 includes a semiconductor line defining a flat-to-sidewall transition 50 having an upwardly extending sidewall 54. As tip 40 scans flat portion 52, the CD-AFM reliably images the sample surface. Thereafter, when tip 40 contacts sidewall as shown in FIG. 3B, the CD-AFM will detect a decrease in the amplitude of oscillation of the probe (e.g., the RMS amplitude of oscillation may go substantially to zero) as distal end 42 contacts sidewall 54, which will operate to cause the controller to generate a signal that is transmitted to the scanner to stop and, in this case, pull up the probe in an attempt to reestablish the setpoint amplitude of oscillation. Alternatively, of course, the sample could be moved depending on the particular scanner arrangement. In this case, typically, tip 40 will stick and the feedback loop (25 in FIG. 1) will be fed signal indicating that the probe needs to be pulled away from the feature being imaged as shown in FIG. 3C. Although the feedback loop or servo command signal to the scanner instructs the scanner to stop, the scanner continues to carry the tip further towards the sidewall 54, as noted above, thus causing poor tracking of the flat-to-sidewall transition. The scanner then attempts to withdraw the tip from the feature being imaged as shown with the arrow marked “W” in FIG. 3C. At some point, typically 10-20 nm up sidewall 54, tip 40 breaks free of sidewall 54.
In this CD mode, the control parameters of oscillating the tip relative to the sample surface are modified to scan sidewall 54 of sample 48 as shown in FIG. 3D. In doing so, however, data is distorted in the flat-to-sidewall transition region given the fact that the system pulls tip 40 upwardly relative to sidewall 54 10-20 nm without taking any data, or alternatively, recording biased data associated with continued movement of scanner (and thus tip 40). This missing or biased data is an artifact in the AFM data and is often referred to as “notching”. This notching phenomenon is illustrated in FIG. 3E, which shows a schematic image 56 generated using conventional CD mode illustrated in FIGS. 3A-3D. Clearly, the notching shown in FIG. 3E is not completely representative of the flat to sidewall transition. Notably, this information is of significant importance to many customers, including semiconductor manufacturers as they attempt to evaluate sample parameters such as line width roughness (LWR) all the way down to the base of the structure.
A more specific illustration of CD mode operation when imaging a feature with a large transition, such as a line of a semiconductor device (30 in FIG. 2), is shown schematically in FIG. 4. Notably, probe motion is described herein, but depending upon the scanner implementation, the sample and/or the tip may be moved.
FIG. 4 illustrates data taken by a conventional CD AFM on a sample 60 where “O's” indicate contact between the tip and sample, for instance, where the RMS amplitude of oscillation goes to zero. At a transition region 62, the scanner is instructed to stop while the momentum of the scanner continues to drive the tip (not shown in FIG. 4) toward a sidewall 64. The tip is served away from the sidewall and moves to a point 68 which may be recorded as a data point as the probe, and specifically the tip, is brought into contact with sidewall 64. In the regions marked 70 in which the tip separates from the sample, in particular sidewall, and the scanner pulls the probe upwardly, no data is typically recorded by the AFM. Most often, the data generated by the “O's” is used to interpolate the data in-between to recreate the vertical sidewall when generating an image of the sample surface. This results in an artifact that is often called “notching.” Though the sidewall can be imaged in this fashion and the data can be interpolated to reasonably accurately represent sidewall 64, the data missing in the region marked “N” still is unacceptable for some applications, i.e., either biased or completely missing causing the above-described “notching” problem. As a result, conventional AFM operation using a CD algorithm required improvement, especially when imaging transition points of features.
To overcome this notching problem, several techniques have been employed. First, when performing AFM metrology, including, for example, in CD mode, there is always a tradeoff between rapidly scanning fast and accurate tracking of the sample, which is in part dictated by the structural bandwidth or closed loop bandwidth of the actuator. Because the response to commanded inputs typically is not immediate, as noted previously, and the momentum of the scanner cannot be halted immediately, the notching problem always remains an issue. However, if the speed of the scan is reduced, the notching effect can be minimized. However, by reducing the scan speed, other issues arise. First, scanning at a slower speed causes the system to be susceptible to stage drift, such as that due to thermal conditions. Secondly, scanning slower means scanning fewer samples per hour. Hence, faster AFM not only provides more accuracy and precision due to the fact that the system is then less susceptible to stage drift and other similar conditions that can compromise the acquired data, it also includes the inherent benefit of being able to image more samples per hour. In the end, because many customer applications require that the associated metrology, including semiconductor metrology, be measured in dollars per measurement, scanning slower can become unacceptable for some customers, to the point that AFM may not be a viable alternative to other known tools for measuring features of samples such as critical dimensions of semiconductor structures.
There are some measures that a user can take to operate an AFM at a slower scan speed, such as using a highly accurate stage and a cooled environment. However, there will always be some motion associated with stopping the scanner, such that, again, notching is nearly always a problem.
Other alternatives to scanning probe microscopy for measuring critical dimensions on semiconductor features, are scanning electron microscopy (SEM) and transmission electron microscopy (TEM, or STEM for scanning). SEM, and even more so TEM, are very high accuracy instruments. However, these techniques are extremely expensive and often difficult to use given the amount of preparation work of the sample required before conducting the measurements. For example, when imaging a semiconductor wafer, SEM and TEM will typically require cutting the wafer at specific locations while preserving structures that are intended to be imaged. This preparation work can take as much as a day or two, and thus will typically slow down the semiconductor fabrication process to analyze the one sacrificial wafer used to analyze the viability of continuing to process that batch of wafers.
Scatterometry is used in the semiconductor fabrication environment, for instance, to measure critical dimensions of sample features, including base dimensions of lines, etc. Scatterometers are extremely fast and can image the types of samples contemplated herein faster than SPMs. Nevertheless, though notching typically is not the problem it is with, for example, AFM, scatterometry includes its own set of limitations. Scatterometry is an indirect measurement that uses data acquired by CD-AFM or CD-SEM to build a library of data that is compared to the measurements obtained by the scatterometer. Techniques such as CD-AFM, SEM, and TEM are thus sometimes referred to as reference metrology systems (RMSs) in that they are tools used by other tools that provide indirect measurements of sample features. Scatterometry, in general, operates by generating a signal that is directed towards a target, and sensing how much of that signal returns from the target after interacting with the target. By comparing that data to a library of data generated by, for example, a CD-AFM, a representation of the target can be generated. Overall, this feature of scatterometry means that significant time-consuming and laborious calibration is required to be able to image samples. A direct method of imaging sample features is therefore preferred.
Moreover, scatterometers only produce average data and thus cannot produce a true RMS reading or a standard deviation of, for example, line width. CD mode AFM, on the other hand, is an adaptive scan that is able to detect in real time what the tip of the AFM is interacting with. Scatterometry is also dependent on properties of the sample due to the fact that different sample properties will interact differently with the source signal generated by the scatterometer.
In sum, the art of high precision and high speed sample measurement, particularly in the semiconductor fabrication environment, was in need of a metrology tool that is capable of a direct measurement of sample properties, such as critical dimensions of semiconductor features, with a high degree of accuracy and repeatability. More particularly, a CD-AFM capable of imaging transition regions, such as the sidewalls of a line, at high speed and without causing artifacts such as notching in the data, was desired.